Metal oxide varistor made by a co-precipation process and freeze-dried

ABSTRACT

A metal oxide varistor is disclosed which a component of grain bodies comprised of zinc oxide and a component of grain boundary layers comprised of another metallic oxide, containing metal other than zinc wherein at least a portion of these starting materials comprised a fine particle powder prepared by a co-precipitatin method. 
     The metal oxide varistor of the present invention is excellent in varistor characteristics such as non-linearity to voltage, life performances and capability of energy dissipation, is small in a scatter of the above characteristics between manufacture lots or within each lot at the time of manufacture, and has a good quality stability. Unexpected results are obtained when the co-precipitated fine particles are subjected to a refrigeration-dehydration type process.

SUMMARY OF THE INVENTION

This invention relates to an oxide varistor, particularly to a zincoxide (ZnO) varistor which is excellent in varistor characteristics suchas non-linearity to voltage, life performances and capability of energydissipation, shows little variation in the above characteristics betweenmanufacture lots or within each lot at the time of manufacture, and hasa good quality stability, and particularly, it relates to an improvementin its materials.

As one of circuit elements made from a semiconductor, there is avaristor, and a varistor made from a zinc oxide sintered body istypically known.

This type of varistor has non-linear voltage-current characteristics,and its resistance decreases abruptly with the raise of the appliedvoltage so that current flowing therethrough increases remarkably.Therefore, such a varistor has been employed practically and widely forabsorption of an extraordinarily high voltage or for stabilization ofvoltage.

Such a zinc oxide varistor as mentioned above is usually manufactured inthe following procedure: Namely, first, a powder of zinc oxide which isa main component is blended, in a predetermined proportion, with a finepowder of a metallic oxide such as bismuth oxide (Bi₂ O₃), antimonyoxide (Sb₂ O₃), cobalt oxide (CoO), manganese oxide (MnO) or the likewhich is an additive component, and these powders are mixed and groundwith the aid of a medium (e.g., zirconia balls) in a suitable mixing andgrinding machine and are then formed, using a suitable binder, intograins each having a predetermined grain diameter. Afterward, a mold ischarged with the above grainy powder, and pressure molding is carriedout to prepare powder compacts (e.g., pellets). The obtained powdercompacts are then sintered at a temperature within the range of 1100° to1350° C. (See, for example, Japanese Journal of Applied Physics, Vol.10, No. 6, June (1976), p. 736 "Nonohmic Properties of Zinc OxideCeramics").

With regard to the obtained sintered bodies, the zinc oxide which is themain component usually constitutes the component of relatively largegrain bodies of as much as several micrometers to several tens ofmicrometers, and the metallic oxide which is the additive componentconstitutes the component of thin grain boundary layers which areinterposed among cantact surfaces of the zinc oxide grain bodies in thestate of wrapping them.

In the zinc oxide varistor which is the sintered body having such a finestructure, a systematic uniformity of the respective components is oneimportant factor for stabilization and improvement of theabove-mentioned various characteristics.

In a conventional manufacturing method, however, it is difficult to givea uniform grain diameter to the zinc oxide powder and the additivecomponent powder which are employed as materials, and since an amount ofthe additive component is generally extremely small as compared withthat of the zinc oxide powder, the mixing of the zinc oxide powder andthe additive component tends to be nonuniform, so that there occurs theproblem that it is very hard to interpose the grain boundary componentlayers each having a uniform thickness among the zinc oxide grainbodies.

Such a matter not only allows the variability of quality properties toincrease between manufacture lots or within one lot of products andbrings about a deterioration in their quality stability,, but also leadsdisadvantageously to a degradation in varistor characteristicsthemselves of the obtained varistor, such as non-linearity to voltage,life performances and capability of energy dissipation.

SUMMARY OF THE INVENTION

Accordingly, an object of this invention is to provide a zinc oxidevaristor in which the respective components are highly fine andparticularly its structure is uniform all over, with the result thatexcellent varistor characteristics can be obtained.

The inventors of this invention have noted the fact that thecharacteristics and reliability of the varistor depend greatly on theuniformity of a grain diameter of each component and the uniformity of athickness of the grain boundary component layers in its structure. Fromthis viewpoint, they have conducted intensive research on a preparationof starting powder materials which permit the acquisition of suchrequirements as mentioned above, and as a result it has been found thatin starting powder materials prepared in a co-precipitation manner whichis widely applied in a process for manufacturing a multicomponentcatalyst, their grain diameter has an extremely small grain diameter andtheir grain diameter distribution is also uniform. Further, they havefound that when the aforesaid starting powder materials are substitutedfor conventional discrete starting powder materials which are previouslyseparately manufactured, the obtained varistor will improve in thevaristor characteristics. And thus, the present invention has beenestablished.

The metal oxide varistor according to this invention comprises acomponent of grain bodies composed of zinc oxide and a component ofgrain boundary layers composed of another metallic oxide, characterizedin that at least a portion of these starting materials is a fineparticle powder prepared by a co-precipitation and freeze drying method.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 and 2 are diagrams showing scatter states between lots andwithin each lot of Samples 1 and 15', respectively, in the Example.

In the varistor according to this invention, the principal component ofthe grain bodies is zinc oxide. As a starting powder material to be usedfor it, a conventional material is acceptable, but a material preparedby the co-precipitation manner mentioned below is preferable.

As the component of the grain boundary layers, any conventionalcompounds are usable, so long as they can form layers among the grainbodies in combination with their zinc oxide component. However,preferable examples of the grain boundary material include one or morekinds of oxides of antimony (Sb), bismuth (Bi), cobalt (Co), manganese(Mn), chromium (Cr), nickel (Ni), silicon (Si), and the like, as well asspinel oxides represented by, for example, Zn₂.33 Sb₀.67 O₄. Among them,oxides of Sb, Bi and Co are more preferred. Particularly, a fineparticle powder of a metallic oxide prepared by co-precipitating atleast one of Sb, Bi and Co with Zn is the most preferable grain boundarylayer component in view of the varistor characteristics.

Now, in the materials for the varistor according to this invention, atleast a portion thereof is prepared in a co-precipitation manner.

For example, the zinc oxide powder for the component of the grain bodiesmay be prepared in accordance with the co-precipitation process, asfollows: First of all, a salt such as Zn(NO₃)₂ is dissolved in apredetermined amount of water to prepare an aqueous solution includingZn²⁺ at a predetermined concentration. Thereto, for example, ammoniawater is added in order to adjust a pH of the whole solution to a levelwithin the range of 6 to 10, so that Zn(OH)₂ precipitates. The resultantprecipitate is collected by filtration, washed with water and dehydratedby means of suction, and a refrigerating dehydration is further carriedout at a low temperature of, for example, -25° C. or less. Afterward,the precipitate is melted, for example, at a temperature of 20° C. orless, extraction water at this time is filtered off, and water is thenremoved therefrom with an alcohol.

The compound Zn(OH)₂ thus obtained in this way is in the state ofusually amorphous powdered grains each having an extremely small graindiameter (0.5 μm or less).

Also, the component of the grain boundary layers can be prepared in likemanner. In this case, procedure is the same as mentioned above exceptthat a salt of a metal of the grain boundary component is used.

With regard to each starting powder material used in this invention, apowder (still in the form of a hydroxide) which has undergone thedehydration treatment as mentioned above may be utilized as it is;alternatively this powder may be subjected to a further dehydration at atemperature within the range of 250° to 300° C. in order to change intoan oxide, and the resultant oxide may be utilized.

In this invention, irrespective of the grain body component (ZnO) andthe grain boundary layer component, at least a portion of the respectivecomponents is prepared by the above-mentioned co-precipitation method.Particularly, with regard to the grain boundary layer component, it ispreferred that at least a portion thereof is prepared in theco-precipitation manner.

In this case, the respective components may be separately prepared asdiscrete precipitates and blended in a predetermined proportion, but itis preferable that the starting powder materials are prepared byprecipitating simultaneously two or more kinds of required components.Further, it is more preferable that all the starting powder materialsfor a metal oxide varistor within the present invention be prepared bythe above-described co-precipitation method.

The co-precipitation of the respective components is preferablyaccomplished by preparing a aqueous solution including metals for therespective metallic oxides in the varistor to be made, at an ionconcentration corresponding to an amount of each metal, and thenco-precipitating the respective components at one time. The reason whythis way is preferred is that the respective precipitates can constitutea co-precipitate in which they coexist in about the same proportion as ametallic composition of the metallic oxides in the varistor to bemanufactured. In other words, according to the above-mentioned manner,the formed co-precipitate contains the respective components in auniform mixing state, so that, when sintered, there can be obtained thevaristor having a system structure in which the respective componentsare uniformly dispersed.

In the varistor according to this invention, the metallic oxide preparedby the co-precipitation process is contained in the whole startingmetallic oxides preferably in an amount of 0.4 to 100% by weight, morepreferably is an amount of 0.4 to 50% by weight.

This invention will be described further in detail in accordance withthe Example as follows:

EXAMPLE A. Preparation of samples

By the use of Zn(NO₃)₂ for Zn, SbCl₃ for Sb, Bi(NO₃)₃ for Bi, Co(NO₃)₂for Co, Mn(NO₃)₂ for Mn, Cr(NO₃)₃ for Cr, Ni(NO₃)₂ for Ni and Na₄ SiO₄for Si, the respective aqueous solutions having predeterminedconcentrations were prepared. The concentrations of the respectivemetallic ions were regulated in terms of corresponding metallic oxides,at blending ratios (mole %) listed in Table 1 in the varistor to bemanufactured. Asterisks in Table 1 are affixed to starting powdermaterials prepared in the co-precipitation manner according to thisinvention.

An aqueous ammonium bicarbonate solution having a concentration of 4Nand ammonia water having the same concentration were added to eachaqueous solution while stirring in order to adjust its pH to 7-8, sothat a precipitate having a grain diameter of less than 0.5 μm wasobtained. Then, each precipitate was collected by filtration, washedwith water and dehydrated by means of suction. The resultant cake wassubjected to a refrigerating dehydration at a temperature of -25° C. orless, and the refrigerated product was melted at 20° C. The extractionwater at this time was filtered off and water was finally removedtherefrom with ethyl alcohol. At the last step, each resultant productwas heated at 300° C. to obtain a starting powder material.

Afterward, the respective starting powder materials were blended in eachratio listed in Table 1 and mixed sufficiently in, for example, a potmade from a nylon resin. After drying of each mixed powder, a suitableamount of PVA was added thereto in order to form its grains.

A mold having a predetermined size and shape was charged with each aboveformed grainy powder, and pressure molding was then carried out. Theresultant pellets were sintered at 1300° C. for 2 hours in order to forma disc a 20 mm in diameter of 2 mm in thickness.

Flame spray electrodes of aluminum were fixed on both the surfaces ofeach disc to provide samples for measurement of characteristics.

Incidentally, in Table 1 below, compounds having no asterisks (*) areconventional starting powder materials.

Further, for comparison, an apostrophe mark is affixed to each samplecomprising material which is similar in a blending ratio to thecorresponding sample without any mark but which was not prepared by theco-precipitation method.

                                      TABLE 1                                     __________________________________________________________________________    Sample Component (Unit: mole %)                                               number ZnO Bi.sub.2 O.sub.3                                                                  Co.sub.2 O.sub.3                                                                  MnO Sb.sub.2 O.sub.3                                                                  NiO Cr.sub.2 O.sub.3                                                                  SiO.sub.2                                  __________________________________________________________________________    Example                                                                        1     94.5*                                                                             0.75*                                                                             0.75                                                                              0.5*                                                                              1.0 1.0*                                                                              0.5*                                                                              1.0                                         2     94.5*                                                                             0.75                                                                              0.75                                                                              0.5*                                                                              1.0 1.0 0.5*                                                                              1.0                                         3     94.5*                                                                             0.75*                                                                             0.75*                                                                             0.5 1.0*                                                                              1.0 0.5 1.0*                                        4     94.5*                                                                             0.75*                                                                             0.75*                                                                             0.5*                                                                              1.0*                                                                              1.0*                                                                              0.5*                                                                              1.0*                                        5     95.75*                                                                            0.5 0.5*                                                                              0.5 1.0*                                                                              1.0 0.25*                                                                             0.5                                         6     95.5*                                                                             0.5*                                                                              0.5*                                                                              0.75*                                                                             0.5 1.0 0.25                                                                              0.5                                         7     95.75*                                                                            0.5 0.5*                                                                              0.75*                                                                             0.5 0.5*                                                                              0.5*                                                                              1.0*                                        8     95.0                                                                              0.5*                                                                              0.5*                                                                              0.5*                                                                              1.0 1.0*                                                                              0.5*                                                                              1.0*                                        9     94.5*                                                                             0.75                                                                              0.75                                                                              0.5 1.0*                                                                              1.0 0.5 1.0                                        10     94.5*                                                                             0.75*                                                                             0.75                                                                              0.5 1.0*                                                                              1.0 0.5 1.0                                        11     94.5*                                                                             0.75*                                                                             0.75*                                                                             0.5 1.0*                                                                              1.0 0.5 1.0                                        12     94.5*                                                                             0.75*                                                                             0.75*                                                                             0.5*                                                                              1.0*                                                                              1.0 0.5 1.0                                        13     96.0*                                                                             0.5 0.5 0.5 1.0*                                                                              0.5*                                                                              0.5 0.5                                        14     95.75*                                                                            0.5*                                                                              0.75                                                                              0.75*                                                                             0.75*                                                                             0.5 0.5*                                                                              0.5                                        15     96.25*                                                                            0.5 0.5 0.75                                                                              0.75*                                                                             0.5 0.25                                                                              0.5                                        16     96.0*                                                                             0.5*                                                                              0.5*                                                                              0.5 0.75*                                                                             1.0 0.25*                                                                             0.5*                                       17     95.75                                                                             0.5*                                                                              0.5 0.5 1.0 0.5 0.5*                                                                              1.0                                        18     95.75                                                                             0.5*                                                                              0.5*                                                                              0.5*                                                                              1.0*                                                                              0.5*                                                                              0.5*                                                                              1.0*                                       19     95.75                                                                             0.5*                                                                              0.5*                                                                              0.5 1.0*                                                                              0.5 0.5*                                                                              1.0                                        20     95.75                                                                             0.5*                                                                              0.5 0.5*                                                                              1.0 0.5 0.5*                                                                              1.0                                        21     96.0                                                                              0.5*                                                                              0.5*                                                                              0.5 0.5*                                                                              1.0*                                                                              0.25*                                                                             1.0                                        22     96.0                                                                              0.75*                                                                             0.5*                                                                              0.75*                                                                             0.5*                                                                              1.0 0.25*                                                                             0.5                                        23     96.25                                                                             0.5*                                                                              0.5 0.5*                                                                              1.0*                                                                              0.5 0.5 0.5                                        24     96.0                                                                              0.5*                                                                              0.5*                                                                              0.75*                                                                             0.5*                                                                              1.0*                                                                              0.5 0.5                                        25     95.0*                                                                             0.5*                                                                              0.5 0.5*                                                                              1.0 1.0*                                                                              0.5*                                                                              1.0                                        26     95.0*                                                                             0.5*                                                                              0.5 0.5 1.0*                                                                              1.0 0.5 1.0                                        27     95.0*                                                                             0.5*                                                                              0.5*                                                                              0.5*                                                                              1.0*                                                                              1.0*                                                                              0.5*                                                                              1.0*                                       28     95.0*                                                                             0.5 0.75*                                                                             0.75                                                                              1.0*                                                                              1.0 0.5 0.5                                        29     96.0                                                                              0.5*                                                                              0.5 0.5*                                                                              0.5 0.5 1.0*                                                                              0.5                                        30     96.5                                                                              0.5*                                                                              0.5*                                                                              0.5*                                                                              1.0 1.0 --  --                                         31     96.5                                                                              0.5*                                                                              0.5 0.5*                                                                              1.0 1.0*                                                                              --  --                                         32     96.5                                                                              0.5*                                                                              0.5 0.5*                                                                              1.0*                                                                              1.0 --  --                                         33     96.5*                                                                             0.5*                                                                              0.5*                                                                              0.5*                                                                              1.0*                                                                              1.0*                                                                              --  --                                         34     96.0*                                                                             0.5 0.75                                                                              0.5 1.25*                                                                             1.0 --  --                                         35     96.0*                                                                             0.5*                                                                              0.75                                                                              0.5*                                                                              1.25                                                                              1.0 --  --                                         36     96.0*                                                                             0.5*                                                                              0.75                                                                              0.5*                                                                              1.25*                                                                             1.0 --  --                                         37     96.0*                                                                             0.5*                                                                              0.75*                                                                             0.5*                                                                              1.25*                                                                             1.0*                                                                              --  --                                         Comparative                                                                   example                                                                         1'   94.5                                                                              0.75                                                                              0.75                                                                              0.5 1.0 1.0 0.5 1.0                                          2'   95.75                                                                             0.5 0.5 0.5 1.0 1.0 0.25                                                                              0.5                                          3'   95.5                                                                              0.5 0.5 0.75                                                                              0.5 1.0 0.25                                                                              0.5                                          4'   95.75                                                                             0.5 0.5 0.75                                                                              0.5 0.5 0.5 1.0                                          5'   95.0                                                                              0.5 0.5 0.5 1.0 1.0 0.5 1.0                                          6'   96.0                                                                              0.5 0.5 0.5 1.0 0.5 0.5 0.5                                          7'   95.75                                                                             0.5 0.75                                                                              0.75                                                                              0.75                                                                              0.5 0.5 0.5                                          8'   96.25                                                                             0.5 0.5 0.75                                                                              0.75                                                                              0.5 0.25                                                                              0.5                                          9'   96.0                                                                              0.5 0.5 0.5 0.75                                                                              1.0 0.25                                                                              0.5                                         10'   96.0                                                                              0.5 0.5 0.5 0.5 1.0 0.25                                                                              1.0                                         11'   96.0                                                                              0.75                                                                              0.5 0.75                                                                              0.5 1.0 0.25                                                                              0.5                                         12'   96.25                                                                             0.5 0.5 0.5 1.0 0.5 0.5 0.5                                         13'   96.0                                                                              0.5 0.5 0.75                                                                              0.5 1.0 0.5 0.5                                         14'   95.0                                                                              0.5 0.75                                                                              0.75                                                                              1.0 1.0 0.5 0.5                                         15'   96.0                                                                              0.5 0.5 0.5 0.5 0.5 1.0 0.5                                         16'   95.75                                                                             0.5 0.5 0.5 1.0 0.5 0.5 1.0                                         17'   96.5                                                                              0.5 0.5 0.5 1.0 1.0 --  --                                          18'   96.0                                                                              0.5 0.75                                                                              0.5 1.25                                                                              1.0 --  --                                         __________________________________________________________________________

B. Measurement of characteristics (1) Life performances

Each sample was placed in a thermostatic chamber, and measurements weremade for initial voltages V_(1mA) and V₁₀μA at the time when currents of1 mA and 10 μA were allowed to flow therethrough, and were further madefor voltages (V_(1mA))₂₀₀ and (V₁₀μA)₂₀₀ at the time when voltages asmuch as 95% of the initial voltages were applied thereto for a period of200 hours. Rates of change [(V_(1mA))₂₀₀ -V_(1mA) ]/V_(1mA) and[(V₁₀μA)₂₀₀ -V₁₀μA ]/V₁₀μA were then evaluated from then and showed interms of percentage (%). This rate of change means that the less it is,the less a characteristic degradation of the sample is.

The rates of change of the respective samples are set forth in Table 2below.

                  TABLE 2                                                         ______________________________________                                         Sample                                                                               ##STR1##                                                                                       ##STR2##                                             number 100[%]           100[%]                                                ______________________________________                                         1     -4.8             -1.0                                                   2     -5.1             -1.1                                                   3     -5.0             -1.2                                                   4     -4.3             -1.1                                                   5     -4.5             -1.3                                                   6     -4.9             -1.2                                                   7     -5.2             -1.2                                                   8     -5.4             -1.3                                                   9     -5.6             -1.5                                                  10     -5.1             -1.3                                                  11     -4.8             -1.2                                                  12     -4.3             -1.2                                                  13     -4.8             -1.3                                                  14     -5.1             -1.5                                                  15     -4.7             -1.4                                                  16     -4.5             -1.2                                                  17     -4.9             -1.2                                                  18     -5.1             -1.1                                                  19     -5.3             -1.0                                                  20     -4.8             -1.0                                                  21     -4.3             -1.5                                                  22     -4.5             -1.6                                                  23     -5.2             -1.1                                                  24     -5.4             -1.2                                                  25     - 4.2            -1.2                                                  26     -4.3             -1.3                                                  27     -3.5             -1.0                                                  28     -4.6             -1.1                                                  29     -5.1             -1.8                                                  30     -4.7             -1.3                                                  31     -4.6             -1.2                                                  32     -4.5             -1.2                                                  33     -4.3             -1.1                                                  34     -4.8             -1.3                                                  35     -4.7             -1.2                                                  36     -4.5             -1.1                                                  37     -4.2             -1.0                                                   1'    -21.5            -5.6                                                   2'    -24.3            -5.1                                                   3'    -25.8            -6.5                                                   4'    -24.9            -5.3                                                   5'    -27.1            -5.4                                                   6'    -25.1            -5.2                                                   7'    -26.2            -5.8                                                   8'    -24.7            -4.9                                                   9'    -23.8            -4.8                                                  10'    -28.1            -6.2                                                  11'    -23.5            -5.8                                                  12'    -29.1            -5.1                                                  13'    -30.3            -5.7                                                  14'    -27.6            -5.6                                                  15'    -25.3            -5.9                                                  16'    -26.2            -5.7                                                  17'    -21.3            -5.6                                                  18'    -21.8            -5.4                                                  ______________________________________                                    

(2) Non-linearity and capability of energy dissipation

A measurement was made for a voltage V₁₀ KA at the time when a currentof 10 KA was allowed to flow through each sample, and a dischargevoltage ratio V_(10KA) /V_(1mA) was evaluated therefrom. This dischargevoltage ratio means that the less it is, the better a non-linearity ofthe sample is. Further, the capability of energy dissipation isrepresented with a rectangular wave discharge bearing capacity (Joul)per unit volume (cm³) of the sample at the time when a currentrectangular wave of 2 m sec is applied thereto, in accordance with theprocedure described on page 43 of JEC-203 (Standard of the JapaneseElectrochemical Committee). The obtained results are set forth in Table3 below.

                  TABLE 3                                                         ______________________________________                                         numberSample                                                                               ##STR3##                                                                               ##STR4##                                               ______________________________________                                         1           1.88     240                                                      2           1.89     250                                                      3           1.87     250                                                      4           1.85     260                                                      5           1.88     240                                                      6           1.90     250                                                      7           1.88     250                                                      8           1.84     260                                                      9           1.95     240                                                     10           1.90     250                                                     11           1.90     250                                                     12           1.87     260                                                     13           1.89     250                                                     14           1.88     240                                                     15           1.87     250                                                     16           1.86     250                                                     17           1.96     240                                                     18           1.86     240                                                     19           1.89     250                                                     20           1.90     240                                                     21           1.91     240                                                     22           1.88     240                                                     23           1.94     240                                                     24           1.88     250                                                     25           1.90     250                                                     26           1.91     250                                                     27           1.86     260                                                     28           1.91     250                                                     29           1.96     230                                                     30           1.93     240                                                     31           1.92     240                                                     32           1.93     240                                                     33           1.91     250                                                     34           1.92     240                                                     35           1.93     240                                                     36           1.92     240                                                     37           1.90     250                                                      1'          1.98     200                                                      2'          1.97     210                                                      3'          2.01     200                                                      4'          1.97     200                                                      5'          1.98     210                                                      6'          1.99     210                                                      7'          2.00     200                                                      8'          1.98     210                                                      9'          2.01     210                                                     10'          1.98     210                                                     11'          1.98     200                                                     12'          1.99     200                                                     13'          2.00     210                                                     14'          1.98     200                                                     15'          2.00     200                                                     16'          1.99     200                                                     17'          2.02     190                                                     18'          2.01     190                                                     ______________________________________                                    

(3) Quality stability of products

With regard to Sample 1, 10 lots at 10 products per lot weremanufactured, and V_(1mA) was measured on all the products to determinetheir variability. The obtained results are exhibited in FIG. 1. Forcomparison, with regard to Sample 15', a similar procedure was carriedout to determine variability for each lot, and the obtained results areexhibited in FIG. 2.

As clearly seen from FIGS. 1 and 2, the samples according to thisinvention are extremely small in the scatter as compared withcomparative samples.

As the above-mentioned results show, the zinc oxide varistor accordingto this invention is excellent in non-linearity (varistorcharacteristics), is great in capability of energy dissipation, is goodin life performances, displays little variability between lots andwithin each lot at the time of manufacture, and is thus excellent in aquality stability. Further, the manufacturing process in this inventionrequires no grinding step, and an inclusion of impurities canaccordingly be prevented completely. Furthermore, it should be notedthat the varistor according to this invention can be obtained with auniform structure.

We claim:
 1. A metal oxide varistor comprising a component of grainbodies comprised of zinc oxide and a component of grain boundary layerscomprised of at least one metallic oxide containing metal other thanzinc, wherein at least a portion of at least one of said zinc oxide andsaid metallic oxide comprises a fine particle powder prepared by aco-precipitation method consisting essentially of the steps of(a)preparing an aqueous solution comprising at least two metal ions; (b) byadding a base, forming a co-precipitate comprising substantially all ofsaid metal ions in the form of corresponding metallic oxides; andthereafter (c) carrying out refrigerating dehydration of saidco-precipitate.
 2. A metal oxide varistor according to claim 1, whereinsaid metal ion in said first aqueous solution comprises at least oneselected from the group consisting of antimony, bismuth, cobalt,manganese, nickel, chromium and silicon.
 3. A metal oxide varistoraccording to claim 1, wherein said fine particle powder prepared by saidco-precipitation method comprises between about 0.4 to about 100% byweight of all metallic oxides comprising said metal varistor.
 4. A metaloxide varistor according to claim 1, wherein at least a portion of bothof said zinc oxide and said metallic oxide comprises a fine particlepowder prepared by said co-precipitation method comprising the stepsof(a) preparing (i) a first aqueous solution comprising at least onemetallic salt containing a metal ion other than zinc and (ii) a thirdaqueous solution comprising a metallic salt containing zinc ion; (b₁)forming a second aqueous solution by homogeneously mixing said first andthird aqueous solution; and (b₂) adding said base to said secondsolution in sufficient amounts to form a co-precipitate from said secondsolution, said co-precipitate comprising substantially all of said zincion and said metal ion other than zinc.
 5. A metal oxide varistor as inclaim 4, wherein said metal ion other than zinc comprises at least oneselected from the group consisting of antimony, bismuth, cobalt,manganese, nickel, chromium, and silicon.
 6. A metal oxide varistoraccording to claim 1, wherein said refrigerating dehydration is carriedout at a temperature of about -25° C. or less.
 7. A process formanufacturing a metal oxide varistor, consisting essentially of thesteps of(a) preparing an aqueous solution comprising at least two ions;(b) by adding a base, forming a co-precipitate comprising substantiallyall of said metal ions in the form of corresponding metallic oxides;then (c) carrying out refrigerating dehydration of said co-precipitate;and (d) molding said co-precipitate under pressure to form a compactbody.
 8. A process according to claim 7, comprising the steps of(a)preparing (i) a first aqueous solution comprising at least one metallicsalt containing a metal ion other than zinc and (ii) a third aqueoussolution comprising a metallic salt containing zinc ion; (b₁) formingsaid second aqueous solution by homogeneously mixing said first andthird aqueous solutions; (b₂) adding said base to said second solutionin sufficient amounts to form a co-precipitate from said secondsolution, said precipitate comprising substantially all of said zinc ionand said metal ion other than zinc; and (c) molding said co-precipitateunder pressure to form a compact body.
 9. A process according to claim 7wherein said refrigerating dehydration is carried out at a temperatureof about -25° C. or less.
 10. A fine particle powder consistingessentially of metallic oxides, formed by a co-precipitation methodconsisting essentially of the steps of(a) preparing an aqueous solutioncomprising at least two metal ions; (b) by adding a base, forming aco-precipitate comprising substantially all of said metal ions in theform of corresponding metallic oxide; and then (c) carrying outrefrigerating dehydration of said co-precipitate,said co-precipitatebeing adapted for manufacturing metallic oxide varistors therefrom. 11.A fine particle powder as in claim 10, wherein said co-precipitationmethod comprises the steps of(a) preparing (i) a first aqueous solutioncomprising at least one metallic salt containing a metal ion other thanzinc and (ii) a third aqueous solution comprising a metallic saltcontaining zinc ion; (b₁) forming a second aqueous solution byhomogeneously mixing said first and third aqueous solutions; and (b₂)adding said base to said second solution in sufficient amounts to form aco-precipitate from said second solution, said co-precipitate comprisingsubstantially all of said zinc ion and said metal ion other than zinc.12. A fine particle powder according to claim 10, wherein saidrefrigerating dehydration is carried out at a temperature of about -25°C. or less.